Patent GB-1028339-A (GB)
Title: Improvements relating to pulse radar equipment
IPC: H04Q3/42
Filed: 1963-01-21
Abstract: 1,028,339. Doppler radar. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Jan. 21, 1963 [Jan. 23, 1962], No. 2604/63. Heading H4D. A Doppler radar comprises means transmitting a plurality of pulse trains at different carrier frequencies and pulse spacings, means comparing, at each carrier frequency, the phase difference between the carrier waves of successive echo pulses with the phase difference between the carrier waves of the corresponding transmitted pulses, means producing a pulse signal if the comparison yields a phase difference above a predetermined amount, and means combining the pulse signals resulting from a plurality of comparisons to produce a resultant pulse signal. The use of a plurality of carrier frequencies and a plurality of pulse spacings makes it much less likely that errors due to record echoes or objects differing in distance by an amount corresponding to a pulse spacing will occur in more than one of the comparison signals, with the result that the resultant signal is largely error free. In the arrangement shown the pulse trains are in the form of trains of pulse pairs A1 and A2, Fig. 1 (a) (not shown), separated by an interval S, the resulting echoes from a single target being P1 and P2. The echoes are received and passed to four discriminators, Fig. 2 (not shown), (four carrier frequencies being used) wherein the frequencies are separated out and the resulting pulses processed. For each pulse train the transmitted pulse pairs are also passed to the discriminator, where they are passed to a phase difference measuring means, Fig. 4, both directly, and through a delay means 1, delaying the pulse pair by an interval S, such that the phase of the carrier wave of the trailing pulse A2 is compared with that of the delayed leading pulse A 1 <SP>1</SP>, Figs. 1(a) and 1(b) (not shown). A signal representing the measured phase difference is fed to a store. The echo pulse pairs P1 and P2 are likewise passed to the phase difference measuring means, directly and indirectly. The signal representing the phase difference between the carrier waves of the trailing echo pulse P2 and the delayed leading echo pulse P 1 <SP>1</SP>, is compared in comparison means with the stored signal and a difference signal is passed via a diode D 3  and threshold means to one input of an AND gate 2. The echo pulse pair is also passed, directly and delayed, to the inputs of an AND gate 1. The AND gate 1 therefore only gives an output at time t 2 , Fig. 1, when pulse P2 coincides with pulse P 1 <SP>1</SP>. The output from the AND gate 1 is fed to the AND gate 2 and since it coincides with the output from the comparison means, the AND gate 2 gives an output. This output is fed to an adder circuit, Fig. 2, via a delay means such that the outputs from all the discriminators arrive at the adder simultaneously. The adder circuit is arranged to only pass on a final output pulse if outputs are received simultaneously from more than a predetermined number of discriminators. The arrangement of the pulse trains is preferably as shown in Fig. 3 (not shown). The pulse spacing in at least one of the trains is preferably variable from pulse to pulse.